1. Field of the Invention
The present invention relates to a method for producing a high-purity BaTiO3 single-crystal piece.
2. Description of the Background Art
Barium titanium oxides are classified into two types according to their crystal structures at room temperature. One type is a perovskite-structured barium titanium oxide (e.g. BaTi3), and the other type is a hexagonal-structured barium titanium oxide. Typically, the hexagonal barium titanium oxide is prepared at a temperature higher than 1700 K by a crystal growth process from a melt or by a solid-phase sintering process. Whereas, the perovskite barium titanium oxide is prepared at a temperature lower than 1700 K by a crystal growth process from a melt or by a solid-phase sintering process. Single crystal growth form a melt usually needs to take more than ten hours and may be contaminated by impurities from container wall. The method of forming a crystalline material using the solid-phase sintering process is difficulty to synthesize a high density sample, and thereby involves a problem about deteriorated characteristic values of a product. For example, as to a maximum relative permittivity of a perovskite-structured barium titanium oxide around room temperature, while the value is about 6000 in the form of a single crystal, it is reduced up to 2000 in the form of a sintered body due to deterioration in density.
Although a perovskite-structured barium titanium oxide prepared by a conventional process has been used for electronic devices, it has a problem about instability in the temperature dependence of dielectric constant that is, a high permittivity at a phase transition temperature is sharply decrease when temperature deviates from the phase transition temperature.
A hexagonal-structured barium titanium oxide has a lower permittivity than that of a perovskite-structured barium titanium oxide. Specifically, a single crystal of the hexagonal-structured barium titanium oxide has a permittivity of 1000 or less at room temperature, and undergoes a phase transition at 74 K a maximum relative permittivity of about several thousand.